Photoelectrochemical properties of the brownmillerite Sr2Fe2O5: Application to electrochemical oxygen evolution

Abstract

The brownmillerite Sr2Fe2O5 is prepared by nitrate route and the physical and photo-electrochemical properties are investigated for the first time. Thermal analysis indicates the formation of the phase at 950°C. A direct optical transition at 0.94 eV, due to Fe3+ splitting in octahedral site, is determined from the diffuse reflectance spectra. The prepared material displays semiconducting properties, and its electrical conductivity follows an exponential type law σ = σoexp (–0.045/kT) with small polaron hopping. The Mott-Schottky plot at pH ~7 is characteristic of n type semiconductor with a flat band potential of 0.40 V SCE and an electron density of 3.6 × 1018 cm–3. The electrochemical impedance spectroscopy shows the predominance of the bulk contribution. The above data are correlated, and the energetic diagram of the junction Sr2Fe2O5/KOH solution is built. The valence band of Sr2Fe2O5 (1.29 V SCE ) is more anodic than the potential of the O2/H2O couple and oxygen is successfully evolved under visible illumination. The best performance (0.17 cm3 O2 (g catalyst)–1 mn–1) occurs at pH ~ 7 with a light-to-chemical energy yield of 0.045% under a light flux of 29 mW cm–2. No deactivation was observed during the second cycle.